Summary of Significant Accounting Policies - Revenue by Vertical Market as Percentage of Total Revenue (Details) - Customer Concentration Risk [Member] - Sales Revenue, Services, Net [Member]	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Concentration Risk [Line Items]
Total revenue	100.00%	100.00%	100.00%
Technology Service Providers [Member]
Concentration Risk [Line Items]
Total revenue	35.20%	31.10%	26.30%
Manufacturing [Member]
Concentration Risk [Line Items]
Total revenue	24.20%	25.70%	24.00%
Healthcare [Member]
Concentration Risk [Line Items]
Total revenue	18.20%	23.50%	28.60%
Financial Services [Member]
Concentration Risk [Line Items]
Total revenue	7.70%	7.10%	7.80%
Energy [Member]
Concentration Risk [Line Items]
Total revenue	5.10%	5.40%	6.10%
General Markets [Member]
Concentration Risk [Line Items]
Total revenue	9.60%	7.20%	7.20%